Yes, not reporting it has great penalties. However, when they do report it the government determines if the treasure has historical importance or not. If it does, the museum is required to pay the finder what the treasure is valued at. If it doesn’t have historical importance, the finder gets it back and do what he wants.
It actually is a law that promotes treasure hunting.
Yes, since they own the property and found the treasure, they are the lawful owners of what they found. I don’t think it is subject to any sort of tax.
And prevents true historical treasures from getting locked away in private collections for decades. It’s a good law.